The present invention relates generally to ion implantation systems, and more specifically to an improved method and system for ion beam containment in an ion beam guide.
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic materials doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in xe2x80x9cn typexe2x80x9d extrinsic material wafers, whereas if xe2x80x9cp typexe2x80x9d extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
For shallow depth ion implantation, high current, low energy ion beams are desirable. In this case, the reduced energies of the ions cause some difficulties in maintaining convergence of the ion beam due to the mutual repulsion of ions bearing a like charge. High current ion beams typically include a high concentration of similarly charged ions which tend to diverge due to mutual repulsion. To maintain low energy, high current ion beam integrity at low pressures, a plasma may be created to surround the ion beam. High energy ion implantation beams typically propagate through a weak plasma that is a byproduct of the beam interactions with the residual or background gas. This plasma tends to neutralize the space charge caused by the ion beam, thereby largely eliminating transverse electric fields that would otherwise disperse the beam. However, at low ion beam energies, the probability of ionizing collisions with the background gas is very low. Moreover, in the dipole magnetic field of a mass analyzer, plasma diffusion across magnetic field lines is greatly reduced while the diffusion along the direction of the field is unrestricted. Consequently, introduction of additional plasma to improve low energy beam containment in a mass analyzer is largely futile, since the introduced plasma is quickly diverted along the dipole magnetic field lines to the passageway chamber walls.
In ion implantation systems, there remains a need for a beam containment apparatus and methodologies for use with high current, low energy ion beams which may be operated at low pressures, and which provides uniform beam containment along the entire length of a mass analyzer beam guide.
The present invention is directed to an apparatus and method for providing a low energy, high current ion beam for ion implantation applications. The invention provides ion beam containment without the introduction of auxiliary plasma and instead enhances beam plasma associated with the ion beam by utilizing the background gas in the beam guide to create the additional electrons required for adequate beam containment. This is accomplished by providing a multi-cusped magnetic field in a beam guide passageway in order to create a magnetic mirror effect in a controlled fashion, as illustrated and described in greater detail hereinafter.
Ion beams propagating through a plasma, such as the beam plasma created by beam interactions with the residual or background gas, reach a steady state equilibrium wherein charges produced by ionization and charge exchange are lost to the beam guide. The remaining plasma density results from a balance between charge formation due to the probability of ionizing collisions, and losses from the beam volume due to repulsion of positive charges by the residual space charge and electron escape as a result of kinetic energy.
Absent plasma enhancement through the introduction of externally generated plasma or enhancement of the beam plasma, the probability for ionizing collisions with the background gas at very low ion beam energies is relatively low. Electrons generated in such a manner are trapped in the beam""s large potential well, orbiting around and through the beam center, interacting with each other by Coulomb collisions, resulting in thermalization of the electron energy distribution. Those electrons in the distribution having an energy greater than the ionization potential of the residual gas molecule have a probability of ionizing such a molecule. The ionizing probability decreases as the electron energy decreases.
In a low energy beam plasma, the majority of the ionization is produced by the trapped electrons. These electrons derive their energy from the center-to-edge beam potential difference, which is the same parameter that causes beam xe2x80x9cblow-upxe2x80x9d. Thus, transportation of low energy ion beams is difficult absent externally generated plasma or enhancement of the beam plasma. Because mass analyzers inherently involve magnetic fields, externally generated plasma fails to diffuse adequately along the arcuate length of a mass analyzer beam guide, instead diffusing quickly along the direction of the magnetic field lines. The use of a multi-cusped magnetic field in accordance with the present invention provides for enhancement of the beam plasma in a low pressure, low energy, high current ion beam system through the controlled creation of a magnetic mirror effect in the passageway.
Additional plasma may also be generated within the ion beam space by electric fields at RF or microwave frequencies. This RF or microwave energy is transferred efficiently to plasma electrons, when a proper magnetic field is present, at a magnitude that yields the ECR condition. The RF or microwave energy may be introduced into the passageway at an appropriate port in the beam guide via any number of coupling methods (e.g., windows, antennas, and the like). Although the dipole magnetic field alone might be employed for the creation of an ECR condition, the selection of the dipole magnetic field strength for a mass analysis magnet is dictated by the momentum of the particle selected for implantation. Consequently, the RF or microwave power source frequency would need to be tuned to that which provides the ECR condition according to the dipole magnetic field strength.
For example, for very low energy Boron beams, the dipole magnetic field is well below the ECR condition at the common 2.45 GHz microwave frequency. Lower frequency energy sources (or variable frequency sources) are available, but are costly. In addition, there is an advantage to using the highest available frequency, as the plasma density limit is proportional to the square of the frequency employed. Thus, the ability to use a high frequency power source in a low energy ion beam application via the selective employment of a controlled multi-cusped magnetic field allows for higher plasma density as well as reduced cost.
According to one aspect of the invention, the apparatus comprises a mass analysis magnet mounted around a passageway along the path of an ion beam, and a magnetic device adapted to provide a multi-cusped magnetic field in the passageway. According to another aspect of the invention, the magnetic device comprises a plurality of magnets mounted along at least a portion of the passageway, whereby the magnets cooperatively interact to provide a magnetic mirror effect along at least a portion of the passageway.
The multi-cusped magnetic field may be superimposed on the dipole field at a specified field strength in a region of the mass analyzer passageway to interact with an electric field of a known RF or microwave frequency for a given low energy ion beam. In this manner, the beam plasma within a mass analyzer dipole magnetic field is enhanced for low energy ion beams without the introduction of externally generated plasma. The RF or microwave energy is efficiently transferred to plasma electrons in the presence of a magnetic field that yields an ECR condition. According to one aspect of the present invention, the ECR condition for a particular ion beam type is dependent upon both the electric field frequency and the magnetic field strength. However, the dipole magnetic field of the mass analysis magnet is typically fixed according to the desired selection of an ion charge-to-mass ratio and the magnitude of the beam energy to be directed to a target wafer.
The other ECR condition variables being thus fixed, an electric field energy source frequency is thus determined. The creation of a multi-cusped magnetic field in the passageway of a mass analyzer according to the present invention advantageously provides localized control over the magnetic field strength within the passageway, which allows use of RF or microwave energy sources at common or commercially available frequencies (e.g., 2.45 GHz). In addition to providing regions of magnetic field strength which satisfy the ECR condition for an appropriate frequency, the multi-cusped magnetic field also increases plasma confinement through a magnetic mirror effect, which significantly enhances the plasma density by reducing losses.
According to another aspect of the invention, the magnetic device may comprise a plurality of longitudinally spaced laterally extending magnets disposed on the top and bottom sides of the mass analyzer beam guide passageway. The magnets may include longitudinally opposite magnetic poles of opposite magnetic polarity, with poles of like polarity on adjacent magnets facing one another, whereby the multi-cusp magnetic field is generated in the passageway. In this manner, the magnetic mirror effect may be established near at least two longitudinally facing magnetic poles of at least two adjacent magnets and spaced from one of the top and bottom sides by a specified distance. The magnets creating the multi-cusped field may thus be designed to create a controlled confinement or containment of a passing ion beam.
According to still another aspect of the invention, an ion implantation system is provided, which comprises an ion source adapted to produce an ion beam along a path and a mass analyzer having an inner passageway. The mass analyzer includes a mass analysis magnet mounted in the inner passageway, and a magnetic device mounted in the inner passageway, wherein the mass analyzer is adapted to receive the ion beam from the ion source and to direct ions of an appropriate charge-to-mass ratio along the path toward a wafer. The magnetic device is adapted to provide a multi-cusped magnetic field in the inner passageway. The magnetic device may comprise a plurality of magnets mounted along at least a portion of the passageway, which generate the multi-cusped magnetic field. The multi-cusped magnetic field enhances the beam plasma, thereby neutralizing the space charge of the ion beam.
According to yet another aspect of the invention, there is provided a method of providing ion beam containment in a low energy ion implantation system. The method comprises producing an ion beam along a longitudinal path using an ion source, providing a mass analyzer having an inner passageway and a mass analysis magnet mounted along the inner passageway, and receiving the ion beam in the mass analyzer from the ion source. The method further comprises directing ions of appropriate charge-to-mass ratio and energy from the mass analyzer along the path toward a wafer, and generating a multi-cusped magnetic field in at least a portion of the passageway using a magnetic device mounted along the passageway.
The plasma enhancement and the resulting beam containment may be further aided by the controlled provision of electric field energy in the passageway of a mass analyzer. Generating this electric field in the passageway may be furthered using a separate waveguide to consistently distribute the electric field energy within the passageway in a controlled fashion. In this manner, the energy distribution may be made more uniform along the longitudinal passageway of the beam guide, allowing creation of electron cyclotron resonance regions throughout the entire length thereof.
According to another aspect of the present invention, there is provided a waveguide for coupling microwave energy from a power source with a beam plasma in a passageway of an ion beam mass analyzer beam guide. The waveguide includes a first dielectric layer surrounded by a metallic coating adapted to propagate microwave energy from the power source throughout the length of the beam guide passageway. The metallic coating may thus form a second and third layer on the top and bottom sides of the first layer. The first layer extends longitudinally along an arcuate path in a first plane from an entrance end to an exit end, and laterally between an inner radial side and an outer radial side. The waveguide further includes laterally extending longitudinally spaced ports or slots through the metallic coating on the side facing the beam guide passageway. The longitudinally spaced ports or slots may be positioned advantageously along the waveguide to correspond to the nodes of a standing wave to thereby effectuate an efficient transfer of power to the beam guide.
In this regard, along the waveguide, a plurality of laterally extending longitudinally spaced magnets may be provided which are adapted to provide a multi-cusped magnetic field in the beam guide passageway. In this way, the multi-cusped magnetic field and the microwave energy from the power source may cooperatively interact to create an electron cyclotron resonance condition along at least a portion of the passageway for beam containment, and the plasma may further be enhanced via the magnetic mirror effect.
According to still another aspect of the invention, a mass analyzer beam guide apparatus is provided for conditioning an ion beam along a path in an ion implantation system. This apparatus comprises a mass analysis magnet mounted in a passageway along the path, a power source adapted to provide an electric field in the passageway, a waveguide adapted to couple the electric field with a beam plasma associated with the ion beam, and a magnetic device adapted to provide a multi-cusped magnetic field in the passageway. Accordingly, the power source, the waveguide, and the magnetic device may be cooperatively adapted to provide containment of the ion beam in at least a portion of the passageway. The beam containment may advantageously be accomplished via an electron cyclotron condition established through the cooperative interaction in the passageway of an RF or microwave electric field powered by the power source and the magnetic device, which may create a multi-cusped magnetic field in the interior of the passageway.
According to yet another aspect of the invention, a waveguide is provided for coupling an electric field with a plasma in an ion beam mass analyzer passageway. The waveguide comprises a base layer located in a first plane adapted to propagate microwave energy from a power source, and having a top, bottom, and lateral metallic layers extending longitudinally along an arcuate path from an entrance end to an exit end and laterally between an inner radial side and an outer radial side. The bottom layer may include a plurality of laterally extending longitudinally spaced ports or slots therethrough between the interior of the passageway and the base layer. Microwave energy from the power source propagating along the base layer is coupled with the plasma in the interior of the passageway near the laterally extending longitudinally spaced ports or slots.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.